A/d converter, d/a converter and voltage source

ABSTRACT

A reference voltage generating circuit generates a reference voltage Vref. An A/D converting circuit compares an analog input voltage Vin with the reference voltage Vref to convert the analog input voltage Vin to a digital output value Dout. A measured value storing circuit stores a measured value of the reference voltage Vref in advance and outputs the stored measured value. A user of an A/D converter corrects the digital output value Dout from the A/D converting circuit by use of the measured value of the reference voltage Vref outputted from the measured value storing circuit, thereby obtaining a digital value representing the analog input voltage Vin accurately irrespective of the accuracy of the reference voltage Vref.

CROSS REFERENCE TO RELATED APPLICATION

This is a Divisional Application, which claims the benefit of pendingU.S. patent application Ser. 10/948,643, filed Sep. 24, 2004. Thedisclosure of the prior application is hereby incorporated herein in itsentirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an A/D converter converting an analoginput voltage to a digital output value, a D/A converter converting adigital input value to an analog output voltage, and a voltage sourceutilizing the D/A converter.

2. Description of the Related Art

An A/D converter compares an analog input voltage with a referencevoltage and converts the analog input voltage to a digital output valuebased on a ratio of the analog input voltage and the reference voltage.For example, a digital output value Dout of a 10-bit A/D converter isexpressed by the following expression (1), where an analog input voltageis Vin and a reference voltage is Vref.Dout=(Vin/Vref)×1024   (1)

A D/A converter converts a digital input value to an analog outputvoltage according to a reference voltage. A voltage source utilizing theD/A converter outputs the analog output voltage from the D/A converteras an output voltage of the voltage source. For example, in an 8-bit D/Aconverter, an analog output voltage Vout is expressed by the followingexpression (2), where a digital input value is Din and a referencevoltage is Vref.Vout=(Din/256)×Vref   (2)

As is apparent from the expressions (1), (2), A/D conversion accuracyand D/A conversion accuracy depend on the accuracy of the referencevoltage. Therefore, with the aim of improving the A/D conversionaccuracy or D/A conversion accuracy a bandgap reference circuit, forexample, is used for a reference voltage generating circuit in order toobtain a highly accurate reference voltage. A potential difference of aPN junction of a semiconductor has negative linear dependency onabsolute temperature under a constant bias current. A potentialdifference between two PN junctions biased by current densitiesdifferent from each other is proportional to the absolute temperature.The bandgap reference circuit utilizes these characteristics to generatea temperature-independent, accurate reference voltage.

Further, Japanese Unexamined Patent Application Publication No.2000-31823 has disclosed an A/D converter capable of providing ahigh-accuracy digital output value even when a reference voltagefluctuates. This A/D converter first compares a digital output valueobtained by A/D converting a voltage of a thermistor dependent on apower source voltage and temperature, with a digital output valueobtained by A/D-converting a voltage of a resistor dependent only onpower source voltage according to the reference voltage, therebycalculating a resistance value of the thermistor. Thereafter, thetemperature at this moment is found using the calculated resistancevalue and a temperature characteristic of the thermistor, to find avoltage of a diode having only temperature dependency by use of thefound temperature. Then, the reference voltage is estimated by anarithmetic operation using the found voltage and a digital output valueobtained by A/D-converting the found voltage according to the referencevoltage. A highly accurate digital output value is obtainable bycorrecting the digital output value from an A/D converting circuitaccording to the estimated reference voltage.

Further, Japanese Unexamined Patent Application Publication No.2000-201076 has disclosed an A/D converter that controls, according toan A/D conversion rate, current consumptions of a comparing circuit foran analog input voltage and a reference voltage and of a referencevoltage generating circuit, and thus is capable of A/D converting at arequired A/D conversion rate with high efficiency without anyunnecessary power consumption.

Meanwhile, if the user of an A/D converter knows a value of a referencevoltage at the time of A/D conversion, he or she can acquire a digitalvalue representing an analog input voltage accurately irrespective ofthe accuracy of the reference voltage, by correcting a digital outputvalue from the A/D converter with use of the value of the referencevoltage at the A/D conversion. However, a standard value of a referencevoltage of the conventional A/D converter is specified but not an actualvalue of the reference voltage. Therefore, the user of the A/D convertercannot know a value of the reference voltage at the time of the A/Dconversion. This makes it necessary for a manufacturer of the A/Dconverter to trim the reference voltage within a predetermined standardrange (a range determined using a standard value as a reference) duringthe manufacturing process of the A/D converter in order to ensure A/Dconversion accuracy. If there is an A/D converter with the referencevoltage of a range not trimmed within the standard range, it is regardedas defective, resulting in a lowered manufacturing yield of A/Dconverters. Such a problem also occurs in D/A converters or voltagesources utilizing the D/A converters.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an A/D converterfrom which a digital value representing an analog input voltageaccurately is obtainable irrespective of the accuracy of a referencevoltage. It is another object of the present invention to provide a D/Aconverter capable of generating an accurate analog output voltageirrespective of the accuracy of a reference voltage. It is still anotherobject of the present invention to provide a voltage source capable ofgenerating an accurate output voltage irrespective of the accuracy of areference voltage. It is yet another object of the present invention toimprove manufacturing yields of A/D converters, D/A converters, andvoltage sources.

According to one of the aspects of the A/D converter of the presentinvention, a reference voltage generating circuit generates a referencevoltage. An A/D converting circuit compares an analog input voltage withthe reference voltage to convert the analog input voltage to a digitaloutput value. A measured value storing circuit stores a measured valueof the reference voltage in advance and outputs the stored measuredvalue. For example, during a manufacturing process of the A/D converter,the reference voltage is measured under a predetermined temperature tostore the measured value in the measured value storing circuit.

A user of the A/D converter can obtain the measured value of thereference voltage corresponding to a value of the reference voltage atthe time of A/D conversion. This allows the user of the A/D converter tocorrect the digital output value from the A/D converting circuit, usingthe measured value of the reference voltage outputted from the measuredvalue storing circuit, and to obtain a digital value representing theanalog input voltage accurately irrespective of the accuracy of thereference voltage. Further, the application of the present inventioneliminates necessity for the manufacturer of the A/D converter to trimthe reference voltage within a standard range during the manufacturingprocess; therefore, they can ship A/D converters with a referencevoltage of a range not trimmed within the standard range as nondefectiveproducts, which have been regarded as defective conventionally. Thisrealizes an improved manufacturing yield of the A/D converters.

In a preferable example of the above-described aspect of the A/Dconverter of the present invention, the measured value storing circuitholds measured values of the reference voltage corresponding totemperatures. A temperature information holding circuit holdstemperature information representing a current ambient temperature ofthe A/D converter. An output control circuit selects, from measuredvalues stored in the measured value storing circuit, a measured valuecorresponding to the temperature information held by the temperatureinformation holding circuit to output the selected measured value.Therefore, the measured value of the reference voltage outputted fromthe A/D converter corresponds to the current ambient temperature of theA/D converter. This allows a user of the A/D converter to constantlyobtain a digital value representing the analog input voltage accuratelyeven when the reference voltage varies with the ambient temperature ofthe A/D converter.

In a preferable example of the above-described aspect of the A/Dconverter of the present invention, a measurement voltage generatingcircuit generates a measurement voltage for temperature measurement thatvaries with the ambient temperature of the A/D converter. A selectingcircuit selects, for output to the A/D converting circuit, themeasurement voltage as the analog input voltage, and thereafter selectsan external input voltage for output. In other words, the A/D convertingcircuit A/D-converts the external input voltage after the measurementvoltage. The temperature information holding circuit holds as thetemperature information the digital output value that is outputted fromthe A/D converting circuit in response to the selection of themeasurement voltage by the selecting circuit. The use of the A/Dconverting circuit for generating the temperature information makes itpossible to generate the temperature information to be held in thetemperature information holding circuit with a simple circuitconfiguration.

In a preferable example of the above-described aspect of the A/Dconverter of the present invention, the measurement voltage generatingcircuit has a first resistor element having temperature independency anda second resistor element having temperature dependency. The firstresistor element and the second resistor element are connected in seriesbetween a supply line of the reference voltage and a ground line. Themeasurement voltage is a voltage of a connecting node between the firstresistor element and the second resistor element. That is, themeasurement voltage varies depending on a temperature characteristic ofthe second resistor element. This facilitates generation of themeasurement voltage that varies with the ambient temperature of the A/Dconverter.

In a preferable example of the above-described aspect of the A/Dconverter of the present invention, a measurement voltage generatingcircuit generates a measurement voltage for temperature measurement thatvaries with the ambient temperature of the A/D converter. A selectingcircuit selects, for output to the A/D converting circuit, themeasurement voltage as an analog input voltage, and thereafter outputsan external input voltage. That is, the A/D converting circuitA/D-converts the external input voltage after the measurement voltage.The temperature information holding circuit holds as the temperatureinformation a digital value which corresponds a temperature found by anexternal control circuit according to the digital output value that isoutputted from the A/D converting circuit in response to the selectionof the measurement voltage by the selecting circuit.

The user of the A/D converter can acquire a temperature characteristicof the reference voltage by obtaining the measured values of thereference voltage in sequence while allowing the external controlcircuit to vary the digital value to be held as the temperatureinformation in the temperature information holding circuit. Therefore,even in a case where there is no available measured value correspondingto the current ambient temperature of the A/D converter, for example, itis possible to correct the digital output value from the A/D convertingcircuit by use of the obtained temperature characteristic of thereference voltage, and to thus acquire a digital value representing theanalog input voltage with higher accuracy. This enables a reduction inthe scale of the measured value storing circuit mounted in the A/Dconverter and a simplification of the circuit configuration of the A/Dconverter.

In a preferable example of the above-described aspect of the A/Dconverter of the present invention, a standard value storing circuitstores therein a standard value of the reference voltage in advance andoutputs the stored standard value. A correcting circuit corrects,according to the measured value and the standard value of the referencevoltage, the digital output value from the A/D converting circuit to adigital value whose basic value is the standard value of the referencevoltage, and outputs the digital value. This allows the user of the A/Dconverter to constantly obtain the digital value representing the analoginput voltage accurately without correcting the digital output valuefrom the A/D converting circuit.

According to one of the aspects of the D/A converter of the presentinvention, a reference voltage generating circuit generates a referencevoltage. A D/A converting circuit converts a digital input value to ananalog output voltage according to the reference voltage. A measuredvalue storing circuit stores therein a measured value of the referencevoltage in advance and outputs the stored measured value. For example,during a manufacturing process of the D/A converter the referencevoltage is measured under a predetermined temperature for storage in themeasured value storing circuit.

A user of the D/A converter can obtain the measured value of thereference voltage corresponding to a value of the reference voltage atthe time of D/A conversion. Therefore, the D/A converting circuit issupplied with the digital input value as a digital value whose basicvalue is the measured value of the reference voltage from the measuredvalue storing circuit, so that it can generate the analog output voltagewith accuracy irrespective of the accuracy of the reference voltage. Inaddition, a manufacturer of the D/A converter need not trim thereference voltage within a standard range during the manufacturingprocess of the D/A converter, and hence, with application of the presentinvention they can ship D/A converters with a reference voltage of arange not trimmed within the standard range as nondefective products,which have been regarded as defective conventionally. This realizes animproved manufacturing yield of the D/A converters.

In a preferable example of the above-described aspect of the D/Aconverter of the present invention, the measured value storing circuitholds measured values of the reference voltage corresponding totemperatures. A measurement voltage generating circuit generates ameasurement voltage for temperature measurement that varies with anambient temperature of the D/A converter. An A/D converting circuitcompares the measurement voltage as an analog input voltage with thereference voltage to convert the measurement voltage to a digital outputvalue. An output control circuit selects from measured values stored inthe measured value storing circuit the measured value corresponding tothe digital output value from the A/D converting circuit to output theselected measured value. Therefore, the measured value of the referencevoltage outputted from the D/A converter corresponds to the currentambient temperature of the D/A converter. This enables constantgeneration of an accurate analog output voltage even when the referencevoltage varies with the ambient temperature of the D/A converter.

In a preferable example of the above-described aspect of the D/Aconverter of the present invention, the measurement voltage generatingcircuit has a first resistor element having temperature independency anda second resistor element having temperature dependency. The firstresistor element and the second resistor element are connected in seriesbetween a supply line of the reference voltage and a ground line. Themeasurement voltage is a voltage of a connecting node between the firstresistor element and the second resistor element. This means that themeasurement voltage varies depending on a temperature characteristic ofthe second resistor element. This facilitates generation of themeasurement voltage that varies with the ambient temperature of the D/Aconverter.

According to one of the aspects of the voltage source of the presentinvention, an output voltage setting circuit is for setting a digitalvalue corresponding to a desired output voltage. A D/A converter usesthe set digital value as a digital input value, and it has a referencevoltage generating circuit, a D/A converting circuit, and a measuredvalue storing circuit. The reference voltage generating circuitgenerates a reference voltage. The D/A converting circuit converts thedigital input value to an analog output voltage according to thereference voltage and outputs the analog output voltage as an outputvoltage of the voltage source. The measured value storing circuit storestherein a measured value of the reference voltage in advance and outputsthe stored measured value. For example, during a manufacturing processof the voltage source, the reference voltage is measured under apredetermined temperature to store the measured value in the measuredvalue storing circuit.

A user of the voltage source can acquire the measured value of thereference voltage corresponding to a value of the reference voltage atthe time of the D/A conversion. Therefore, it is possible to generatethe output voltage with accuracy irrespective of the accuracy of thereference voltage by setting, with the output voltage setting circuit, adigital value whose basic value is the measured value of the referencevoltage outputted from the measured value storing circuit. Further, amanufacturer of the voltage source need not trim the reference voltagewithin a standard range during the manufacturing process of the voltagesource; therefore, with the application of the present invention theycan ship voltage sources with a reference voltage of a range not trimmedwithin the standard range as nondefective products, which have beenconsidered defective conventionally. This realizes an improvedmanufacturing yield of the voltage sources.

In a preferable example of the above-described aspect of the voltagesource of the present invention, the D/A converter includes the measuredvalue storing circuit storing therein measured values of the referencevoltage corresponding to temperatures, a measurement voltage generatingcircuit, an A/D converting circuit, and an output control circuit. Themeasurement voltage generating circuit generates a measurement voltagefor temperature measurement that varies with an ambient temperature ofthe voltage source. The A/D converting circuit compares the measurementvoltage as an analog input voltage with the reference voltage to convertthe measurement voltage to a digital output value. The output controlcircuit selects, from measured values stored in the measured valuestoring circuit, a measured value corresponding to the digital outputvalue from the A/D converting circuit to output the selected measuredvalue. Therefore, the measured value of the reference voltage outputtedfrom the voltage source corresponds to the current ambient temperatureof the voltage source. This enables constant generation of an accurateoutput voltage even when the reference voltage varies with the ambienttemperature of the voltage source.

In a preferable example of the above-described aspect of the voltagesource of the present invention, the measurement voltage generatingcircuit has a first resistor element having temperature independency anda second resistor element having temperature dependency. The firstresistor element and the second resistor element are connected in seriesbetween a supply line of the reference voltage and a ground line. Themeasurement voltage is a voltage of a connecting node between the firstresistor element and the second resistor element. That is, themeasurement voltage varies depending on a temperature characteristic ofthe second resistor element. This facilitates generation of themeasurement voltage that varies with the ambient temperature of thevoltage source.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a block diagram of a first principle of the A/D converter ofthe present invention;

FIG. 2 is a block diagram of a second principle of the A/D converter ofthe present invention;

FIG. 3 is a block diagram of a third principle of the A/D converter ofthe present invention;

FIG. 4 is a block diagram of a fourth principle of the A/D converter ofthe present invention;

FIG. 5 is a block diagram of a first principle of the D/A converter ofthe present invention;

FIG. 6 is a block diagram of a second principle of the D/A converter ofthe present invention;

FIG. 7 is a block diagram of a first principle of the voltage source ofthe present invention;

FIG. 8 is a block diagram of a second principle of the voltage source ofthe present invention;

FIG. 9 is a block diagram showing a first embodiment of the A/Dconverter of the present invention;

FIG. 10 is a circuit diagram showing an example of a bandgap referencecircuit;

FIG. 11 is a block diagram showing a second embodiment of the A/Dconverter of the present invention;

FIG. 12 is a block diagram showing a third embodiment of the A/Dconverter of the present invention;

FIG. 13 is a block diagram showing a forth embodiment of the A/Dconverter of the present invention;

FIG. 14 is a block diagram showing a first embodiment of the D/Aconverter of the present invention;

FIG. 15 is a block diagram showing a second embodiment of the D/Aconverter of the present invention;

FIG. 16 is a block diagram showing a first embodiment of the voltagesource of the present invention; and

FIG. 17 is a block diagram showing a second embodiment of the voltagesource of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described,using the drawings.

FIG. 1 shows a first basic principle of the A/D converter of the presentinvention. An A/D converter 10 has a reference voltage generatingcircuit 11, an A/D converting circuit 12, and a measured value storingcircuit 13. The reference voltage generating circuit 11 generates areference voltage Vref to output it to the A/D converting circuit 12.The A/D converting circuit 12 compares an analog input voltage Vin withthe reference voltage Vref to convert the analog input voltage Vin to adigital output value Dout and outputs the digital output value Dout. Themeasured value storing circuit 13 stores a measured value of thereference voltage Vref in advance and outputs the stored measured value.

FIG. 2 shows a second basic principle of the A/D converter of thepresent invention. The same reference numerals and symbols are used todesignate the same elements as the elements described in FIG. 1, anddescription thereof will not be given. An A/D converter 20 has areference voltage generating circuit 11, an A/D converting circuit 12, ameasured value storing circuit 13 holding a plurality of measured valuesof a reference voltage Vref corresponding to a plurality oftemperatures, a measurement voltage generating circuit 21, a selectingcircuit 22, a temperature information holding circuit 23 for holdingtemperature information representing a current ambient temperature ofthe A/D converter 20, and an output control circuit 24.

The measurement voltage generating circuit 21 generates a measurementvoltage Vm for temperature measurement that varies with the ambienttemperature of the A/D converter 20. For example, the measurementvoltage generating circuit 21 has a first resistor element R1 havingtemperature independency and a second resistor element R2 havingtemperature dependency. The first resistor element R1 and the secondresistor element R2 are connected in series between a supply line of thereference voltage Vref and a ground line. The measurement voltage Vm isa voltage of a connecting node N between the first resistor element R1and the second resistor element R2. Therefore, the measurement voltageVm varies according to the temperature characteristic of the secondresistor element R2.

As an analog input voltage Vin of the A/D converting circuit 12, theselecting circuit 22 selects an external input voltage VEin for outputafter the measurement voltage Vm. In other words, the A/D convertingcircuit 12 A/D-converts the external input voltage VEin after themeasurement voltage Vm. The temperature information holding circuit 23holds as temperature information a digital output value Dout that isoutputted from the A/D converting circuit 12 as a result of theselection of the measurement voltage Vm by the selecting circuit 22. Theoutput control circuit 24 selects from measured values stored in themeasured value storing circuit 13 a measured value corresponding to thetemperature information held by the temperature information holdingcircuit 23 to output the selected measured value.

FIG. 3 shows a third basic principle of the A/D converter of the presentinvention. The same reference numerals and symbols are used to designatethe same elements as the elements described in FIG. 1 and FIG. 2, anddescription thereof will not be given. An A/D converter 30 is the sameas the A/D converter 20 in FIG. 2 except that it has a temperatureinformation holding circuit 31 in place of the temperature informationholding circuit 23. The temperature information holding circuit 31 holdsas temperature information a digital value corresponding to atemperature that is found by an external control circuit 32 based on adigital output value Dout that is outputted from the A/D convertingcircuit 12 as a result of the selection of a measurement voltage Vm by aselecting circuit 22.

FIG. 4 shows a fourth basic principle of the A/D converter of thepresent invention. The same reference numerals and symbols are used todesignate the same elements as the elements described in FIG. 1, anddescription thereof will not be given. An A/D converter 40 is configuredby adding a standard value storing circuit 41 and a correcting circuit42 to the A/D converter 10 in FIG. 1. The standard value storing circuit41 stores therein a standard value of a reference voltage Vref inadvance and outputs the stored standard value to the correcting circuit42. According to a measured value of the reference voltage Vref from themeasured value storing circuit 13 and the standard value of thereference voltage Vref from the standard value storing circuit 41, thecorrecting circuit 42 corrects a digital output value Dout from the A/Dconverting circuit 12 to a digital value whose basic value is thestandard value of the reference voltage Vref, and outputs the digitalvalue.

FIG. 5 shows a first basic principle of the D/A converter of the presentinvention. A D/A converter 50 has a reference voltage generating circuit51, a D/A converting circuit 52, and a measured value storing circuit53. The reference voltage generating circuit 51 generates a referencevoltage Vref to output it to the D/A converting circuit 52. The D/Aconverting circuit 52 converts a digital input value Din to an analogoutput voltage Vout according to the reference voltage Vref to outputthe analog output voltage Vout. The measured value storing circuit 53stores a measured value of the reference voltage Vref in advance andoutputs the stored measured value.

FIG. 6 shows a second basic principle of the D/A converter of thepresent invention. The same reference numerals and symbols are used todesignate the same elements as the elements described in FIG. 5, anddescription thereof will not be given. A D/A converter 60 has areference voltage generating circuit 51, a D/A converting circuit 52, ameasured value storing circuit 53 holding a plurality of measured valuesof a reference voltage Vref corresponding to a plurality oftemperatures, a measurement voltage generating circuit 61, an A/Dconverting circuit 62, and an output control circuit 63.

The measurement voltage generating circuit 61 generates a measurementvoltage Vm for temperature measurement that varies with the ambienttemperature of the D/A converter 60. For example, the measurementvoltage generating circuit 61 has a first resistor element R1 havingtemperature independency and a second resistor element R2 havingtemperature dependency. The first resistor element R1 and the secondresistor element R2 are connected in series between a supply line of thereference voltage Vref and a ground line. The measurement voltage Vm isa voltage of a connecting node N of the first resistor element R1 andthe second resistor element R2. Therefore, the measurement voltage Vmvaries according to the temperature characteristic of the secondresistor element R2. The A/D converting circuit 62 compares themeasurement voltage Vm as an analog input voltage with the referencevoltage Vref to convert the measurement voltage Vm to a digital outputvalue Dout and outputs the digital output value Dout to the outputcontrol circuit 63. The output control circuit 63 selects from measuredvalues stored in the measured value storing circuit 53 a measured valuecorresponding to the digital output value Dout from the A/D convertingcircuit 62 to output the selected measured value.

FIG. 7 shows a first basic principle of the voltage source of thepresent invention. The same reference numerals and symbols are used todesignate the same elements as the elements described in FIG. 5, anddescription thereof will not be given. A voltage source 70 has an outputvoltage setting circuit 71 and the D/A converter 50 in FIG. 5. A digitalvalue corresponding to a desired output voltage is set at the outputvoltage setting circuit 71. The D/A converter 50 uses the set digitalvalue as a digital input value Din. The D/A converting circuit 52 of theD/A converter 50 outputs an analog output voltage Vout as an outputvoltage of the voltage source 70.

FIG. 8 shows a second basic principle of the voltage source of thepresent invention. The same reference numerals and symbols are used todesignate the same elements as the elements described in FIG. 5 to FIG.7, and description thereof will not be given. A voltage source 80 has anoutput voltage setting circuit 71 and the D/A converter 60 in FIG. 6.The D/A converter 60 uses a digital value set at the output voltagesetting circuit 71 as a digital input value Din. The D/A convertingcircuit 52 of the D/A converter 60 outputs an analog output voltage Voutas an output voltage of the voltage source 80.

FIG. 9 shows a first embodiment of the A/D converter of the presentinvention. An A/D converter 100 is, for example, a 10-bit A/D converterformed as a semiconductor integrated circuit chip, and it has areference voltage generating circuit 102, a selecting circuit 104, anA/D converting circuit 106, a 10-bit register 108, a ROM 110 (a measuredvalue storing circuit), a 10-bit register 112, and a control circuit 114controlling the whole A/D converter 100.

The reference voltage generating circuit 102 generates a referencevoltage Vref to output it to the A/D converting circuit 106. Thereference voltage generating circuit 102 is constituted of a knownbandgap reference circuit BGR, for example, as shown in FIG. 10. Thebandgap reference circuit BGR stably outputs a bandgap voltage Vbgr(approximately 1.2 V) of silicon independently of the ambienttemperature.

The selecting circuit 104 selects one of external input voltages Vin0 toVinn according to an instruction from the control circuit 114 to outputit as an analog input voltage Vin to the A/D converting circuit 106.According to an instruction from the control circuit 114, the A/Dconverting circuit 106 compares the analog input voltage Vin outputtedfrom the selecting circuit 104 with the reference voltage Vref outputtedfrom the reference voltage generating circuit 102 to convert the analoginput voltage Vin to a 10-bit digital output value Dout and outputs thedigital output value Dout to the register 108.

Every time the A/D converting circuit 106 performs A/D conversion, forexample, the register 108 accepts the digital output value Doutoutputted therefrom. A register value of the register 108 is readablevia an external terminal. Therefore, a user (system) of the A/Dconverter 100 is able to obtain the digital output value Dout (theresult of the A/D conversion) from the A/D converting circuit 106 byreading the register value of the register 108.

The ROM 110 is a nonvolatile memory such as a fuse or EEPROM, and itstores a measured value (10-bit digital value) of the reference voltageVref in advance and outputs the stored measured value to the register112. Incidentally, the A/D converter 100 has, for example, a monitor padfor monitoring the reference voltage Vref, and a write pad and a writecircuit for data write to the ROM 110, though they are not shown in thedrawing. At the probe inspection during a manufacturing process of theA/D converter 100, the measured value of the reference voltage Vref isobtained via the monitor pad under a predetermined temperature, andwritten to the ROM 110 via the write pad and the write circuit.

The register 112 accepts the measured value of the reference voltageVref outputted from the ROM 110, for example, upon power-on reset of theA/D converter 100. A register value of the register 112, similarly tothat of the register 108, is readable via an external terminal.Therefore, a user of the A/D converter 100 can obtain the measured valueof the reference voltage Vref by reading the register value of theregister 112.

For example, when a standard value of the reference voltage Vref is 5.0V, a value of the analog input voltage Vin is 1.25 V, and a value of thereference voltage Vref at the time of A/D conversion is 4.9 V, thedigital output value Dout from the A/D converting circuit 106 (theregister value of the register 108) is given by the expression (1) as261. Further, the measured value of the reference voltage Vref outputtedfrom the ROM 110 (the register value of the register 112) is 1003representing 4.9 V. Therefore, a user of the A/D converter 100 obtains adigital value (256) representing the analog input voltage Vin (1.25 V)accurately by correcting the digital output value Dout (261) from theA/D converting circuit 106 according to a ratio of the standard value(1024) of the reference voltage Vref and the measured value (1003).

On the other hand, with regard to a conventional A/D converter, that is,one having no ROM 110, since the user cannot know the value (4.9 V) ofthe reference voltage Vref at the time of A/D conversion, he or shecannot correct the digital output value Dout (261) from the A/Dconverting circuit 106. This consequently creates an error between anactual value of the analog input voltage Vin and a value the user of theA/D converter knows.

As described above, in this embodiment, the user of the A/D converter100 uses the measured value of the reference voltage Vref obtained byreading the register value of the register 112 to correct the digitaloutput value Dout from the A/D converting circuit 106 obtained byreading the register value of the register 108, so that the user is ableto obtain the digital value representing the analog input voltage Vinaccurately (namely, a desired external input voltage of the externalinput voltages Vin0 to Vinn). Further, a manufacturer of the A/Dconverter 100 need not trim the reference voltage Vref within a standardrange during a manufacturing process of the A/D converter 100, so thatthey can ship A/D converters with a reference voltage Vref of a rangenot trimmed within the standard range as nondefective, which hasconventionally been dealt as defective. This can enhance a manufacturingyield of the A/D converter 100.

FIG. 11 shows a second embodiment of the A/D converter of the presentinvention. The same reference numerals and symbols are used to designatethe same elements as the elements described in the first embodiment ofthe A/D converter, and detailed description thereof will not be given.Similarly to the A/D converter 100 of the first embodiment, an A/Dconverter 200 is, for example, a 10-bit A/D converter formed as asemiconductor integrated circuit chip, and it has a reference voltagegenerating circuit 102, a selecting circuit 204, an A/D convertingcircuit 106, a register 108, a ROM 210 (a measured value storingcircuit, an output control circuit), a 10-bit register 212, a controlcircuit 214 controlling the whole A/D converter 200, a 10-bit register216 (a temperature information holding circuit), and an externalhigh-accuracy resistor R and thermistor Th (a measurement voltagegenerating circuit).

The high-accuracy resistor R (a first resistor element) and a thermistorTh (a second resistor element) are connected in series between a supplyline of a reference voltage Vref and a ground line. A voltage of aconnecting node N of the high-accuracy resistor R and the thermistor This outputted as a measurement voltage Vm to the selecting circuit 204.The high-accuracy resistor R has temperature independency, in otherwords, a resistance value thereof is substantially constant irrespectiveof the ambient temperature of the A/D converter 200. The thermistor Thhas temperature dependency, in other words, a resistance value thereofchanges in accordance with the ambient temperature of the A/D converter200. This means that the measurement voltage Vm varies according to thetemperature characteristic of the thermistor Th, or it varies with theambient temperature of the A/D converter 200. Such a measurement voltageVm can be expressed by the following expression (3).Vm={Th/(R+Th)}×Vref   (3)

According to an instruction from the control circuit 214, the selectingcircuit 204 selects one of the measurement voltage Vm and external inputvoltages Vin0 to Vinn to output the selected voltage as an analog inputvoltage Vin to the A/D converting circuit 106. The control circuit 214instructs the selecting circuit 204 to select the measurement voltage Vmfirst in response to an A/D conversion request, and then to select adesired one from the external input voltages Vin0 to Vinn. The controlcircuit 214 also instructs the A/D converting circuit 106 to A/D convertalong with the instruction to the selecting circuit 204.

The register 216 accepts a digital output value Dout outputted from theA/D converting circuit 106 every time the A/D converting circuit 106 A/Dconverts the measurement voltage Vm. The digital output value Doutresulting from the A/D conversion of the measurement voltage Vm (aregister value of the register 216) is expressed by the expression (4)as a transformation of the expressions (1), (3). Since a resistancevalue of the high-accuracy resistor R may be regarded as a constant, thedigital output value Dout resulting from the A/D conversion of themeasurement voltage Vm depends only on the resistance value of thethermistor Th, that is, on the ambient temperature of the A/D converter200. Therefore, this digital output value Dout can be used astemperature information representing the ambient temperature of the A/Dconverter 200.Dout={Th/(R+Th)}×1024   (4)

Similarly to the ROM 110 of the first embodiment, the ROM 210 is anonvolatile memory such as a fuse or EEPROM, and it stores a pluralityof measured values (10-bit digital values) of the reference voltage Vrefcorresponding to temperatures in advance. The ROM 210 selects a measuredvalue corresponding to the temperature represented by the register valueof the register 216 to output it to the register 212. Incidentally,similarly to the A/D converter 100 of the first embodiment, the A/Dconverter 200 has, for example, a monitor pad for monitoring thereference voltage Vref, a write pad and a write circuit for data writeto the ROM 210, though they are not shown in the drawing. At the probeinspection during a manufacturing process of the A/D converter 200, theplural measured values of the reference voltage Vref are obtained undervarious temperature conditions via the monitor pad, and written to theROM 210 via the write pad and the write circuit.

The register 212 accepts the measured value of the reference voltageVref outputted from the ROM 210, for example, every time the A/Dconverting circuit 106 A/D converts each of the external input voltagesVin0 to Vinn. A register value of the register 212 is readable via anexternal terminal. The measured value of the reference voltage Vrefaccepted by the register 212 corresponds to the digital output valueDout resulting from the latest A/D conversion of the measurement voltageVm, in other words, it corresponds to the current ambient temperature ofthe A/D converter 200. Therefore, a user of the A/D converter 200obtains the measured value of the reference voltage Vref correspondingto the current ambient temperature of the A/D converter 200 by readingthe register value of the register 212.

With the above-described structure, the same effects as those of thefirst embodiment of the A/D converter are obtainable also in thisembodiment. In addition, the measured value of the reference voltageVref obtained by reading the register value of the register 212corresponds to the current ambient temperature of the A/D converter 200.This allows a user of the A/D converter 200 to constantly obtain thedigital value representing the analog input voltage Vin accurately evenwhen the reference voltage Vref varies with the ambient temperature ofthe A/D converter 200.

FIG. 12 shows a third embodiment of the A/D converter of the presentinvention. The same reference numerals and symbols are used to designatethe same elements as the elements described in the first and secondembodiments of the A/D converter, and detailed description thereof willnot be given. An A/D converter 300 has a 10-bit register 316 (atemperature information holding circuit) in place of the register 216 ofthe A/D converter 200 of the second embodiment. The rest ofconfiguration of the A/D converter 300 is the same as that of the A/Dconverter 200 of the second embodiment. Data write to the register 316is possible via an external terminal. Further, the A/D converter 300 isconnected to a microcontroller 390 (an external control circuit) on asystem board.

In the A/D converter 300 as configured above, for example, themicrocontroller 390 reads a register value of a register 108 resultingfrom A/D conversion of a measurement voltage Vm (a digital output valueDout from an A/D converting circuit 106), and calculates a resistancevalue of a thermistor Th by the expression (4), using the read registervalue. Then, the calculated resistance value and the temperaturecharacteristic of the thermistor Th are used to find the current ambienttemperature of the A/D converter 300, and a digital value correspondingto the found temperature is written to the register 316. In responsethereto, a ROM 210 outputs a measured value corresponding to the currentambient temperature of the A/D converter 300 to a register 212.

With the above-described structure, the same effects as those of thefirst and second embodiments of the A/D converter are obtainable also inthis embodiment. In addition, in order to obtain the temperaturecharacteristic of the reference voltage Vref, a user of the A/Dconverter 300 sequentially obtains measured values thereof outputtedfrom the ROM 210 by reading register values of the register 212 whileallowing the microcontroller 390 to vary the digital value to be writtento the register 316. Consequently, in a case where the measured valuecorresponding to the current ambient temperature of the A/D converter300 is not stored in the ROM 210, for example, it is possible to correctthe digital output value Dout from the A/D converting circuit 106,using-the obtained temperature characteristic of the reference voltageVref, and to thus obtain the digital value representing an analog inputvoltage Vin with higher accuracy. This makes it possible to reduce thenumber of measured values to be stored in the ROM 210, in other words,the capacity of the ROM 210, simplifying the circuit configuration ofthe A/D converter 300.

FIG. 13 shows a fourth embodiment of the A/D converter of the presentinvention. The same reference numerals and symbols are used to designatethe same elements as the elements described in the first embodiment ofthe A/D converter, and detailed description thereof will not be given.An A/D converter 400 is configured by adding a ROM 418 (a standard valuestoring circuit) and a correcting circuit 420 to the A/D converter 100of the first embodiment. Similarly to the ROM 110, the ROM 418 is anonvolatile memory such as a fuse or EEPROM, and it stores a standardvalue (10-bit digital value) of a reference voltage Vref in advance andoutputs the stored standard value to the correcting circuit 420. Forexample, at the probe inspection during a manufacturing process of theA/D converter 400, the standard value of the reference voltage Vref iswritten to the ROM 418 as well as a measured value thereof is written tothe ROM 110.

According to the measured value from the ROM 110 and the standard valuefrom the ROM 418, the correcting circuit 420 corrects a register valueread from the register 108, namely, a digital output value Dout from theA/D converting circuit 106, to a digital value Dout′ whose basic valueis the standard value of the reference voltage Vref, and outputs thedigital value Dout′. The digital value Dout′ after the correction isexpressed by the following expression (5), where a measured value of thereference voltage Vref is X1 and a standard value of the referencevoltage Vref is X2. Therefore, the correcting circuit 420 can be easilyconfigured with a multiplying circuit and a dividing circuit.Dout′=(X2/X1)×Dout   (5)

With the above-described structure, the same effects as those of thefirst embodiment of the A/D converter are obtainable also in thisembodiment. In addition, since the digital output value Dout from theA/D converting circuit 106 (the register value read from the register108) is corrected inside the A/D converter 400, a user of the A/Dconverter 400 can constantly obtain the digital value Dout′ representingan analog input voltage Vin accurately without correcting the digitaloutput value Dout from the A/D converting circuit 106.

FIG. 14 shows a first embodiment of the D/A converter of the presentinvention. A D/A converter 500 is, for example, an 8-bit D/A converterformed as a semiconductor integrated circuit chip, and it has areference voltage generating circuit 502, an 8-bit register 504, a D/Aconverting circuit 506, a ROM 508 (a measured value storing circuit),and an 8-bit register 510. The reference voltage generating circuit 502generates a reference voltage Vref to output it to the D/A convertingcircuit 506. The reference voltage generating circuit 502 is structuredby using a known bandgap reference circuit BGR, for example, as shown inFIG. 10. Data write to the register 504 is possible via an externalterminal. The register 504 outputs a register value thereof as a digitalinput value Din of the D/A converting circuit 506.

In response to data write to the register 504, the register 504 outputsthe digital input value Din. The D/A converting circuit 506 receives andconverts the digital input value Din to an analog output voltageaccording to the reference voltage Vref outputted from the referencevoltage generating circuit 502, to output the analog output voltage as adesired one of external output voltages Vout0 to Voutn. The ROM 508,which is a nonvolatile memory such as a fuse or EEPROM, stores ameasured value (8-bit digital value) of the reference voltage Vref inadvance to output the stored measured value to the register 510.Incidentally, the D/A converter 500 has, for example, a monitor pad formonitoring the reference voltage Vref, a write pad and a write circuitfor data write to the ROM 508, though they are not shown in the drawing.At the probe inspection during a manufacturing process of the D/Aconverter 500, the measured value of the reference voltage Vref isobtained via the monitor pad under a predetermined temperature, andwritten to the ROM 508 via the write pad and the write circuit.

The register 510 accepts the measured value of the reference voltageVref outputted from the ROM 508, for example, upon power-on reset of theD/A converter 500. A register value of the register 510 is readable viaan external terminal. Therefore, a user of the D/A converter 500 is ableto obtain the measured value of the reference voltage Vref by readingthe register value of the register 510.

With the D/A converter 500 as configured above, the user reads theregister value of the register 510 to obtain the measured value of thereference voltage Vref corresponding to a value of the reference voltageVref at the time of D/A conversion. Then, the user of the D/A converter500 writes to the register 504 a digital value whose basic value is themeasured value of the reference voltage Vref, so that the externaloutput voltages Vout0 to Voutn (analog output voltages) are accuratelygenerated irrespective of the accuracy of the reference voltage Vref.

As described above, in this embodiment, the D/A converting circuit 506is supplied, as the digital input value Din, with the digital valuewhose basic value is the measured value of the reference voltage Vrefoutputted from the ROM 508, so that the external output voltages Vout0to Voutn can be accurately generated irrespective of the accuracy of thereference voltage Vref. In addition, a manufacturer of the D/A converter500 need not trim the reference voltage Vref within a standard rangeduring a manufacturing process of the D/A converter 500; therefore, theycan ship D/A converters with a reference voltage Vref of a range nottrimmed within the standard range as nondefective products, which havebeen conventionally regarded as defective. This can enhance amanufacturing yield of the D/A converter 500.

FIG. 15 shows a second embodiment of the D/A converter of the presentinvention. The same reference numerals and symbols are used to designatethe same elements as the elements described in the first embodiment ofthe D/A converter, and detailed description thereof will not be given.Similarly to the D/A converter 500 of the first embodiment, a D/Aconverter 600 is, for example, an 8-bit D/A converter formed as asemiconductor integrated circuit chip, and it has a reference voltagegenerating circuit 502, a register 504, a D/A converting circuit 506, aROM 608 (a measured value storing circuit, an output control circuit),an 8-bit register 610, an A/D converting circuit 612, an 8-bit register614, and an external high-accuracy resistor R and thermistor Th (ameasurement voltage generating circuit).

The high-accuracy register R and the thermistor Th are connected inseries between a supply line of a reference voltage Vref and a groundline. A voltage of a connecting node N of the high-accuracy resistor Rand the thermistor Th is outputted as a measurement voltage Vm to theA/D converting circuit 612. The high-accuracy register R has temperatureindependency, in other words, its resistance value is substantiallyconstant irrespective of the ambient temperature of the D/A converter600. The thermistor Th has temperature dependency, that is, itsresistance value varies according to the ambient temperature of the D/Aconverter 600. Therefore, the measurement voltage Vm varies according tothe temperature characteristic of the thermistor Th, in other words, itvaries according to the ambient temperature of the D/A converter 600.

In response to an A/D conversion request, the A/D converting circuit 612compares the measurement voltage Vm as an analog input voltage with thereference voltage Vref to convert the measurement voltage Vm to adigital output value Dout, and outputs the digital output value Dout tothe register 614. The register 614 accepts the digital output value Doutoutputted from the A/D converting circuit 612 every time the A/Dconverting circuit 612 A/D converts. The register 614 outputs a registervalue thereof to the ROM 608. Similarly to the ROM 508 of the firstembodiment, the ROM 608 is a nonvolatile memory such as a fuse orEEPROM, and it stores a plurality of measured values (8-bit digitalvalues) of the reference voltage Vref corresponding to temperatures inadvance. The ROM 608 selects a measured value corresponding to atemperature represented by the register value of the resistor 614 tooutput the selected measured value to the register 610. Incidentally,similarly to the D/A converter 500 of the first embodiment, the D/Aconverter 600 has, for example, a monitor pad for monitoring thereference voltage Vref, a write pad and a write circuit for data writeto the ROM 608, though they are not shown in the drawing. At the probeinspection during a manufacturing process of the D/A converter 600, theplural measured values of the reference voltage Vref are obtained viathe monitor pad under various temperature conditions, and written to theROM 608 via the write pad and the write circuit.

The register 610 accepts the measured value of the reference voltageVref outputted from the ROM 608, for example, every time the A/Dconverting circuit 612 A/D converts. A register value of the register610 is readable via an external terminal. The measured value of thereference voltage Vref accepted by the register 610 corresponds to thedigital output value Dout resulting from the latest A/D conversion ofthe measurement voltage Vm, in other words, it corresponds to thecurrent ambient temperature of the D/A converter 600. Therefore, a userof the D/A converter 600 reads the register value of the register 610 toobtain the measured value of the reference voltage Vref corresponding tothe current ambient temperature of the D/A converter 600.

With the above-described structure, the same effects as those of thefirst embodiment of the D/A converter are obtainable also in thisembodiment. In addition, the measured value of the reference voltageVref obtained by reading the register value of the register 610corresponds to the current ambient temperature of the D/A converter 600.This enables constant generation of accurate external output voltagesVout0 to Voutn, even when the reference voltage Vref varies with theambient temperature of the D/A converter 600.

FIG.16 shows a first embodiment of the voltage source of the presentinvention. The same reference numerals and symbols are used to designatethe same elements as the elements described in the first embodiment ofthe D/A converter, and detailed description thereof will not be given. Avoltage source 700 has an output voltage setting circuit 702 and the D/Aconverter 500 of the first embodiment. A user of the voltage source 700sets digital value corresponding to a desired output voltage to theoutput voltage setting circuit 702. The output voltage setting circuit702 writes the set digital value to the register 504 of the D/Aconverter 500. Therefore, the D/A converter 500 uses the digital valueset at the output voltage setting circuit 702 as a digital input valueDin. The D/A converting circuit 506 of the D/A converter 500 outputsexternal output voltages Vout0 to Voutn as output voltages of thevoltage source 700.

As described above, this embodiment enables generation of accurateoutput voltages irrespective of the accuracy of the reference voltageVref by setting at the output voltage setting circuit 702 a digitalvalue whose basic value is the measured value of a reference voltageVref obtained by reading a register value of the register 510. Inaddition, a manufacturer of the voltage source 700 need not trim thereference voltage Vref within a standard range during a manufacturingprocess of-the voltage source 700, so that they can ship voltage sourceswith a reference voltage Vref of a range not trimmed within the standardrange as nondefective products, which have been regarded as defectiveconventionally. This can enhance a manufacturing yield of the voltagesources 700.

FIG. 17 shows a second embodiment of the voltage source of the presentinvention. The same reference numerals and symbols are used to designatethe same elements as the elements described in the first and secondembodiments of the D/A converter and the first embodiment of the voltagesource, and detailed description thereof will not be given. A voltagesource 800 has an output voltage setting circuit 702 and the D/Aconverter 600 of the second embodiment. The output voltage settingcircuit 702 writes a set digital value to the register 504 of the D/Aconverter 600. Therefore, the D/A converter 600 uses the digital valueset to the output voltage setting circuit 702 as a digital input valueDin. The D/A converting circuit 506 of the D/A converter 600 outputsexternal output voltages Vout0 to Voutn as output voltages of thevoltage source 800.

As described above, the same effects as those of the first embodiment ofthe voltage source are obtainable also in this embodiment. In addition,a measured value of a reference voltage Vref is obtained by reading aregister value of the register 610, and it corresponds to the currentambient temperature of the voltage source 800. This enables constantgeneration of accurate output voltages even when the reference voltageVref varies with the ambient temperature of the voltage source 800.

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and scope ofthe invention. Any improvement may be made in part or all of thecomponents.

1. A D/A converter comprising: a reference voltage generating circuitgenerating a reference voltage; a D/A converting circuit converting adigital input value to an analog output voltage according to thereference voltage; and a measured value storing circuit storing thereina measured value of the reference voltage in advance and outputting thestored measured value.
 2. The D/A converter according to claim 1,further comprising: a measurement voltage generating circuit generatinga measurement voltage for temperature measurement that varies with anambient temperature of the D/A converter; an A/D converting circuitcomparing the measurement voltage as an analog input voltage with thereference voltage to convert the measurement voltage to a digital outputvalue; and an output control circuit selecting, from measured valuesstored in said measured value storing circuit, a measured valuecorresponding to the digital output value from said A/D convertingcircuit, and outputting the selected measured value, wherein saidmeasured value storing circuit stores therein measured values of thereference voltage, the measured values corresponding to temperatures. 3.The D/A converter according to claim 2, wherein: said measurementvoltage generating circuit has a first resistor element and a secondresistor element connected in series between a supply line of thereference voltage and a ground line, the first resistor element havingtemperature independency, the second resistor element having temperaturedependency; and the measurement voltage is a voltage of a connectingnode between the first resistor element and the second resistor element.4. A voltage source comprising: an output voltage setting circuit forsetting a digital value corresponding to a desired output voltage; and aD/A converter using the set digital value as a digital input value,wherein said D/A converter comprises: a reference voltage generatingcircuit generating a reference voltage; a D/A converting circuitconverting the digital input value to an analog output voltage accordingto the reference voltage to output the analog output voltage as anoutput voltage of the voltage source; and a measured value storingcircuit storing therein a measured value of the reference voltage inadvance and outputting the stored measured value.
 5. The voltage sourceaccording to claim 4, wherein: said measured value storing circuitstores therein measured values of the reference voltage, the measuredvalues corresponding to temperatures; and said D/A converter furthercomprises: a measurement voltage generating circuit generating ameasurement voltage for temperature measurement that varies with anambient temperature of said voltage source; an A/D converting circuitcomparing the measurement voltage as an analog input voltage with thereference voltage to convert the measurement voltage to a digital outputvalue; and an output control circuit selecting, from measured valuesstored in said measured value storing circuit, a measured valuecorresponding to the digital output value from said A/D convertingcircuit, and outputting the selected measured value.
 6. The voltagesource according to claim 5, wherein: said measurement voltagegenerating circuit has a first resistor element and a second resistorelement connected in series between a supply line of the referencevoltage and a ground line, the first resistor element having temperatureindependency, the second resistor element having temperature dependency;and the measurement voltage is a voltage of a connecting node betweenthe first resistor element and the second resistor element.